The mechanism by which specific cell-mediated immunity is suppressed during the course of Mycobacterium lepraemurium (MLM) infection of mice will be investigated. Two types of suppressor mechanism will be examined: that mediated by lymphocytes (presumably T cells) and that mediated by humoral factors (immune complexes or idiotypic antibody). Advantage will be taken of the specific cross-reactive immunity that exists between MLM and M tuberculosis. Thus, the effector and suppressor factors produced during the course of MLM infection will be evaluated by measuring their effects upon M. tuberculosis in mice.